Electrical Troubleshooting Quiz — Dec. 21, 2021

The plant has a varnishing system that involves multiple machines, and about 40% of the parts produced in this plant go through that varnishing system.

It begins with three large “dip and spin” machines. Such a machine dips the parts into hot varnish. From the dip and spin, the parts move along an open-air track overhead so the varnish can set and lose its tackiness. The parts are then dropped off the track and collected into bins.

Operators mount the parts on a rack that is then put onto a track that that feeds a drying oven. Each rack goes into the oven in a batch. The operator punches in the code for that batch, to get the correct “recipe” for baking time and temperature, usually depending on which dip and spin they came from.

Upon exiting the oven, the rack is robotically separated from its track and moved to a cooling area. This area has fans blowing from underneath into a ducted hood. The waste heat exits the plant through roof vents.

Sharon was the systems engineer for this machine, but she quit more than 10 months ago, and the company has not been able to hire a replacement. One of the reasons she quit was her frustration with this varnishing system. It was always breaking down somewhere. She had made several recommendations for design changes but could not get funding approval.

The major headaches include:

• The dip and spin sometimes stays dipped. Operators have to use a metal rod to “goose it” into motion. About half the time, that doesn’t work because the motor for that function blows its overloads.
• The dip and spin sometimes won’t spin. The operators use the same solution as with the stuck dipping arm.
• The parts are held by electromagnets and never fall off in the varnish vat. But a couple of times a day, one falls off when going through the open-air track. The typical part makes quite a racket as it clangs its way down.

At the oven stage, problems are rare. However, problems upstream interfere with the baking schedule, causing pileups of parts in the oven inbound area. Clutter management problems have resulted in accidentally mixing parts that require different recipes. Some parts had discolored varnish from overbaking, and others had “weak” varnish from underbaking.

It looks like an expensive upgrade project is not in the cards. But the downtime and other issues are costly, so the plant manager wants to see some kind of big improvement that can be paid for from existing operations or maintenance budgets. If you can deliver, he will talk with HR about hiring you into Sharon’s old job. It’s a big pay increase and a good career move.

Where might you focus?

Check these areas first

Perhaps the weight and viscosity of the varnish weren’t accounted for when determining the motor selections. It is almost certain that the torque curve of the motors do not match the counterpart curve for the load. An easy fix is to upgrade the motor to the next size up. The extra torque should eliminate the problems with overload.

To ensure you are correcting the actual problem, use an ammeter with high/low recording to measure the current draw on each motor.

Don’t jump to that easy fix just yet, however. There may be an even easier one. Check the branch circuit for voltage imbalance. If it’s more than a few percent, go to the panel and look for any resistance heaters or lights run from the same panel. Rebalance as needed to reduce the voltage imbalance. Unless you decide to set a new transformer and panel to separate these loads, this work comes out of the normal maintenance budget.

But wait. We still are not ready to jump to that easy fix. Check each motor supply (branch or feeder) circuit for the power factor. If you have all those coils (motor windings) with no capacitance to offset the shift, your power factor will be low, which means those motors will draw more current and perhaps enough to open their overloads. Installing a power factor correction capacitor at each of these loads can be done on the maintenance budget.

You probably think that we are now ready to jump to that easy fix. One more thing you need to check (and this is outside your electrical training) is the varnish. Contact the manufacturer to find out how it is supposed to be stored, how long it can be used in an open-top vat before becoming too gooey, and other things you should know about it. For example, what is the correct method for cleaning the dipping vat when changing varnish? Next, provide this information to operations and ask them to compare it to what they actually do.

After all this, you are finally ready to jump to that easy fix. However, it’s also an opportunity to get a fairly inexpensive upgrade to go with it. A larger motor provides more torque. If the other fixes did not make these problems go away, this should do the trick. If not, change the gear ratio in the gearbox to multiply the torque produced by the motor. You may want to combine this with a power factor-corrected, harmonics-corrected variable-frequency drive (VFD).

Power circuit connection

What about the falling parts? Given the low frequency of this failure, you have a bad connection in the power circuit to one of the magnet heads. A quick way to fix this is to re-do each connection in that circuit. In a system like this, the connection is probably on a terminal strip inside a control cabinet. Use a torquing screwdriver when remaking the connection. Often in a panel, some terminal strip connections are overtorqued and others are undertorqued. If practical, perform a thermal imaging scan of this cabinet before the work and another such scan an hour after the work is completed. Compare the two.

Before submitting a proposal for new motors, VFDs, or other equipment, write out the math involved in not going ahead with the repair. This is what will convince the bean counters to approve the purchases you are asking approval for.

Use historical repair data to add up all the repair costs over the past year. Ask operations to do something similar for the revenue losses and any other costs on their end. Now you have a total that greatly exceeds the amount you are asking for.